1. Field of the Invention
The present invention relates to an optical apparatus which is provided with an autofocus (AF) function, such as a video camera.
2. Description of the Related Art
Optical apparatuses provided with an AF function including a video camera mainly employ an automatic focusing method (hereinafter referred to as “an. AF method”) in which part of a video signal (high-frequency components of a video signal) indicating sharpness of an object is extracted and evaluated to adjust the focus of an image-taking optical system. An example of operation in the AF method of this type is described with reference to FIG. 11.
FIG. 11 is a block diagram showing the structure of a video camera. In FIG. 11, reference numeral 101 shows the video camera, 102 shows a focus lens included in an image-taking optical system, not shown, and 103 shows an image-pickup device which converts light from the image-taking optical system into an electrical signal for output as a video signal.
Reference numeral 104 shows a video signal processing circuit which processes the video signal output from the image-pickup device 103, 105 shows an AF evaluating circuit which calculates a degree of sharpness as an AF evaluation value based on the processed video signal, and 106 shows a control circuit which calculates a target position of the focus lens 102 based on the degree of sharpness input thereto from the AF evaluating circuit 105.
Reference numeral 107 shows a motor which drives the focus lens 102 in accordance with a control signal from the control circuit 106, and 108 shows an object distance detector which detects the position of the focus lens 102 and outputs a feedback signal indicating the current position of the focus lens 102 to the control circuit 106.
Light passing through the focus lens 102 forms an image on an image-pickup plane of the image-pickup device 103. The image-pickup device 103 photoelectrically converts the object image and outputs the resulting video signal. The video signal processing circuit 104 processes the video signal input thereto from the image-pickup device 103 through a filter or the like to optimize the video signal. The AF evaluating circuit 105 calculates the degree of sharpness as an AF evaluation value in a focus detecting area defined in the center of an image-taking area. Since the focus detecting area is defined in the center of the image-taking area, the image-taking optical system is always focused on an object positioned in the center of the image-taking area.
The object distance detector 108 is mechanically coupled to the focus lens 102 to detect the position of the focus lens 102 and output a position feedback signal to the control circuit 106. The control circuit 106 calculates a target position of the focus lens 102 based on the degree of sharpness calculated by the AF evaluating circuit 105 and the position feedback signal provided by the object distance detector 108, and outputs a position control signal to the motor 107.
The motor 107 drives the focus lens 102 in accordance with the position control signal input thereto from the control circuit 106. In this manner, the automatic focusing control of the focus lens 102 is performed.
The video camera shown in FIG. 11, however, suffers from the following disadvantages since it is not provided with a function of selecting the focus detecting area or a function of storing the selected focus detecting area.
Specifically, the focus detecting area is always positioned in the center of the image-taking area, so that when an image of an object needs to be taken off the center in the image-taking area, either of two approaches must be taken, that is, the automatic focusing control is not activated and the focus lens is manually adjusted, or the automatic focusing control is activated and the video camera is panned or tilted to position the object in the center of the image-taking area. As a result, the operation for the focusing is complicated and other operations may be adversely affected.